4 Discussion
4.1 Proliferation conditions for shrubs in desert steppe
Grazing is one of the main factors influencing the dynamics of grassland shrubs (Van Auken, 2000; 2009). The overgrazing hypothesis suggests that chronic overgrazing reduces the biomass of herbaceous plants, which leads to increased resource availability for shrubs, resulting in shrubs becoming the dominant vegetation (Archer et al., 1995; Van De Koppelet al., 2002; Tefera et al., 2007). Our findings showed that when the sheep grazing is introduced or the stocking rate increased, Stipa breviflora desert steppe will not be encroached by shrubs. This finding does not support the hypothesis that chronic overgrazing is a driver of grassland shrub proliferation in the desert steppe. Zhang et al. (2006) studied the responses of Caragana microphylla morphological characteristics to different stocking rates in a typical steppe in Inner Mongolia, and found that the height of Caragana microphylladecreased significantly with increasing stocking rate, which suggested that shrubby plants may resist herbivore feeding via individual miniaturization (Wang et al., 2010). The results of these studies are consistent with the results of our experiment, and support the hypothesis that grazing inhibited encroachment of desert steppe by shrubs (Zhang et al., 2014). Therefore, sheep introduction can prevent the proliferation of shrubs in the desert steppe, while removal of sheep grazing can release shrubs from top-down controls. Sheep grazing is one of barriers limiting shrub encroachment in desert steppe.